Process for the electrolytic deposition of metallic chromium



.tion of Metallic Chromium,

'way of Patented June 16, 1925.

g UNITED STATES 1,542,549 PATENT OFFICE...

RICHARD GRAB, or SHEFFIELD, ENGLAND, assrenon r0 ELECROM LIMITED, or

, g c ,snnrrrnLn, ENGLAND. C

No Drawing. Application filed April 2,

To all whom it may concern: 4

Be it known that I, Rrormno GRAH, of Shefiield, England, a citizen of Germany, have invented a certain new and useful Improved Process for the Electrolytic Deposiof which the following is a specification.

This invention relates to .an improved process of electrolytically depositing metallic chromium from chromium compounds, and more especially to a type of process of electrolytic deposition of chromium by means of a bath which contains a mixture of chromic oxide and an acid and used in the proportions by weight of acid and oxide of chromium approximately equal or a smaller quantity of chromium oxide but not less than half the weight of the acid, the relative proportion being as far as possible maintained constant during the operation, and to which acid is added and t e temperature kept constant.

It has previously been proposed to deposit chromium from a solution of chromic sulphate, chromic anhydride and boric acid also being present in the solution, and also from a solution of chromic and boric or tartaric acid with an average current density of 0.20 amperes er square centimetre at a pressure of 20 v0 ts. Chromium has previousl been deposited from a solution of the sulp ates but in this process avery high current density is required, say of between 2 and 5 amps. per sq. cm. at a pressure of 3 to 6 volts but the'deposit resulting is brittle and easily flakes ofi'.

A process of the type described according to this invention is characterized in that the ac1d added to the chromium oxide 1S hydrochloric acid or acetic acid, with the addition of a suitable conduct-ing acid or salt, for example, boric acid, tartaric acid, citric acid, or sulphuric acid, which produces a non-colloidal solution.

In the above process, in the preferred form, the proportion by weight of hydrochloric acid or acetic acid to chromium oxide of approximately 2 to 1 has been found to give a good result. This strength of the solution requires to be maintained during the working of the solution, and one doing this is to add fresh materials to the solution, while working with suitable anodes. The additional subsidiary conducting acids, such as boric acid, tartaric acid,

rnoonss roarnn ELECTROLYTIC nnrosr'rioivon METALLIC omtomrum.

1924. Serial No. 703,602.

citric acid, or sulphuric acid are used in small quantlties.

While a proportion of 2 to 1 is maintained,

an increase in current density employed has no appreciable effect on the amount of metal deposlted in a given time. If, however, the proportion employed is 4 to 3 or still higher, 1. -e.,. if the proportion of chromium oxide present in the solution is increased, better results are obtained. The solution is worked at a temperature of 18 to 20 centigrade, using a current density of 2 to 3 amperes per square decimetre and a voltage of 2 to 4. Working the solution as described above,

a coating can be deposited on many metals commonly used for commercial purposes, any desired thickness being obtainablef This process enables chromium to be deposited in such a condition that it can be highly polished.

What I claim and desire to secure by Letters Patent is 1. A. process of electrolytically depositng metallic chromium consisting in providing a bath containing a mixture of chromic oxide and a non-oxidizing acid in the proportions by weight of oxide of chromium more than half the weight of the acid, maintainlng the relative proportions substantially constant during the operation, adding a suitable conducting acid which produces a non-colloidal solution, and maintaining the temperature constant.

2. A process of electrolytically depositnig metallic chromium consisting in providing a bath containing a mixture of chromic oxide and a non-oxidizing acid and used in the proportions by weight of oxide of chromium more than half the weight of the acid, maintaining the relative proportions substantially constant during the operation, adding a suitable conducting acid which produces a non-colloidal solution, maintaining the temperature constant, and supplying current density of about 2 to 3 amperes per square decimetre at a constant temperature of about 18 degrees centigrade and a voltage of not less than 2 and not more than 4.

3. A process of electrolytically depositing metallic chromium consisting in providing a bath containing a mixture of chromic oxide and a non-oxidizing acid and used in the proportions by weight of acid and oxide of chromium approximately 4 to 3, maintaining the relative proportions substantially constant during the operation, adding a sultable conducting acid which produces a non-colloidal solution and maintaining the temperature constant.

4. A process of-electrolytlcally depos ting metallic chromium consisting 1n providing a bath containing a mixture of chromlc oxide and a non-oxidizing acid and used in the proportions by weight of oxide of chromium more than half the weight of the acid, maintaining the relative proportions substantially constant during the operation, adding boric acid, and maintaining the tempera ture constant.

5. A process for electrolytically depositing metallic chromium conslstlng in roviding a bath containing a mixture of c om1c oxide and acetic acid used in the proportions by weight of oxide of chromium more than half the weight of the acid, maintaining the relative proportions substantially constant during the operation, adding a suitable conductin acid which produces a non-colloidal solution, and maintaining the temperature constant.

6. A process for electrolytically depositing metallic chromium consisting in providing a bath containing a mixture of chromic oxide and acetic acid used in the proportions by weight of oxide of chromium more than half the weight of the acid, maintaining the relative proportions substantially constant during the operation, boric acid and maintaining the temperature constant.

In testimony whereof, I aflix my signature.

RICHARD GRAH.

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